which is a bounded one from \(L_p(\mathbb {C})\)  to itself for \(p\in (1, \infty )\)  by the celebrated theorem of Calderon and Zygmund [1]}.
We denote by \(\Vert K\Vert _p\)  the norm of the operator \(K\)  in \(L_p(\mathbb {C})\)  and similarly we denote the norms of operators acting in \(L_p(U)\)  with
arbitrary domain \(U\) . It is important for us that \(\Vert K\Vert _p\rightarrow 1\)  when \(p\rightarrow 2\) , see [2]}, [3]}.
